Sunbrella is the industry leader in marine fabrics. They have a proven track record.
It can be bought from upholstery/canvas suppliers, if you are a business. If you are in CA, for example, you must have a state tax number, so you can collect sales tax for the state. If you live in a state that has no sales tax,like Oregon where I live, they will sell to you sans tax credentials. In SF Bay area, I use Kronke Canvas. They've always been my choice for marine fabrics and canvas goods.
I also have used Top Gun, for fishing boat covers and such. It's main advantage over Sunbrella is it has a higher abrasion rating. It is some really tough stuff. Not as many colors as Sunbrella. Stiffer, and denser. More of a rough hand to it. Some canvas shops use it for abrasion patches under Sunbrella.
I used to buy a lot of my supplies from Silver State Suppliers in Salt Lake City, UT. THey carry an extensive line of all kinds of fabrics.
Another, slightly less consumer-friendly source is Keyston Bros. They have most anything automotive and marine you need. I never cared for working with them, though. But, they are one of the biggest.

Check at some of the upholstery shops. They often have Sunbrella remnants they will sell. Tell them you need it for 'patch material', as you are heading out on a voyage. You might not get 10-12 yards of it, but you can usually pick up a few from them.